1. Technical Field
The present invention relates to a light-emitting device capable of converting electric energy to light, particularly an organic electroluminescent device, hereinafter on occasion referred to as a “device”, a “light-emitting device”, an “EL device” or an “organic EL device”.
2. Related Art
Organic electroluminescent (EL) devices have been attracting attention as promising display devices for obtaining luminescence of a high intensity at a low driving voltage. An important characteristic value of an organic electroluminescent device is external quantum efficiency. External quantum efficiency is calculated by “external quantum efficiency φ=number of photons discharged from a device/number of electrons injected into a device”. The higher the value of external quantum efficiency, the lower the power consumption of the device, and thereby a device having a higher value of external quantum efficiency is advantageous.
The external quantum efficiency of the organic electroluminescent device is determined by “external quantum efficiency φ=internal quantum efficiency×light extraction efficiency”. In an organic EL device which uses fluorescence from an organic compound, the limit value of the external quantum efficiency is considered to be about 5% since the limit value of internal quantum efficiency is 25% and light extraction efficiency is about 20%.
A device which uses a triplet light-emitting material (phosphorescent light emitting material) has been reported as a method for improving the external quantum efficiency of the device by enhancing the internal quantum efficiency of the organic electroluminescent device (for instance, see WO 00/70655). In comparison with a conventional device using fluorescence (singlet light-emitting device), this device can enhance external quantum efficiency and a maximum value of external quantum efficiency of 8% has been attained (external quantum efficiency at 100 cd/m2 is 7.5%). However, since a phosphorescent light emission from a heavy atomic metal complex is used, the response of light emission has been slow, and an improvement in durability also remains to be desired.
A singlet light-emitting device that uses energy translation from triplet excitons to singlet excitons has been reported as a method for alleviating this problem (for instance, see WO 01/8230).
However, the maximum value of external quantum efficiency of the device described in the document is 3.3%, and this does not exceed the external quantum efficiency (φ=5%) of a conventional singlet light-emitting device. There is, therefore, scope for further improvement in the device.
It is accordingly an object of the present invention to provide a light-emitting device with a superior light-emitting efficiency.